This invention relates in general to gas operated firearms and deals more particularly with an improved gas operating mechanism for a firearm. More specifically, the invention is concerned with an improved gas operating mechanism of an annular type which is coaxially supported on a portion of a firearm barrel to move axially to the barrel in response to pressure generated by gases of explosion. Such a gas operating mechanism is illustrated and described in my U.S. Pat. No. 3,988,964 for GAS OPERATED FIREARM WITH METERING ADJUSTMENT, issued Nov. 2, 1976, and includes a generally cylindrical piston slidably supported on a firearm barrel and which partially defines an annular gas chamber surrounding an associated portion of the barrel. The gas chamber is further defined by a manually adjustable regulator positionable axially along the barrel to meter the escape of gases of explosion from the bore into the gas chamber. A variety of ammunition of a specific caliber is available for a firearm of the aforedescribed general type. However, differences in bullet weight, powder type and quantity influence the operational characteristics of the firearm. Other variables which effect the stability of ammunition, including weather conditions, also influence operational characteristic of a weapon, but are somewhat less predictable. While a manually regulated gas operating mechanism of the aforedescribed general type may be adjusted to compensate for such variables the possibility of human error is ever present. Failure to adjust the gas regulator or an error in judgment in making an adjustment may adversely affect the firing characteristic of a weapon producing excessive recoil with resulting loss of accuracy. Accordingly, it is the general aim of the present invention to provide a gas operating mechanism for a firearm which includes an automatic pressure regulator which compensates for variations in pressure generated by the gases of exploding in firing a weapon and vents the gases of explosion from the gas operating mechanism when excessive gas pressure develops therein.